1. Field of the Invention
The invention relates to the regulation and control of degasification of a hydraulic circuit.
2. Description of Prior Art
Hydraulic circuits generally require a liquid which is void of any dissolved or suspended gases. In effect, gases in suspension, particularly air, result in disturbances in the operation of the system, such as cavitation in the pumps, pressure jumps, defects in regulation, strange noises, etc. Entrapped gas even results in serious deterioration of the elements in the circuit, e.g., erosion by virtue of cavitation, corrosion resulting from the abrasion of materials etc. The dissolved gases can become liberated during storage or in the course of the liquid being heated. Finally, the gases may concentrate in a pocket at certain high points of the circuit thus introducing an impedance within the circuit. Thus, it is often necessary or at least desirable to periodically de-gas the liquid circulating through a hydraulic circuit.
Degasification becomes indispensible when considering hydraulic circuits used in airplanes for reasons of safety. First, these circuits comprise components which are very sensitive to an absence of homogeneity in the liquid. Furthermore, in modern airplanes, the two tank stages (high and low pressure) are entirely closed and air, leaking in very small quantities but in a continuous stream, enters through the connections and the safety joints and cannot be separated from the oil in the tank as partially occurs in conventional tanks where gas exists above the level of the liquid. Finally, the response times of elements controlled by the circuit are directly influenced by the impedance of the circuit and must remain within precise limits.
The degasification of the hydraulic circuit of airplanes is normally performed at the same time as its purification and its testing which are performed by means of a test bench. The test bench comprises an auxiliary hydraulic circuit comprising essentially one pump, filters, and a relatively large volume tank, and is connected in parallel across the circuit being checked. The pump of the auxiliary circuit is provided with its own driving means. The oil of the circuit to be checked passes permanently through the tank of the auxiliary circuit and the degasification is performed by connecting the high point of the tank of the auxiliary circuit to a vacuum pump and involves both the circuit being checked and the auxiliary circuit. In other words, such degasification occurs exactly in the manner of a simple hydraulic circuit with a conventional reservoir.
It is always important to known whether the degasification is operating adequately or not. In the case of a permanent degasification of a simple circuit one can appropriately vary the degree of vacuum created in the tank by the vacuum pump. In the case of a periodic degasification on a test bench with an auxiliary circuit, one thus determines the moment when the operation may be stopped.
Presently, in order to verify that the quantity of gas contained in the liquid is sufficiently low, one inserts into the return line of the tank a transparent area which is appropriately lit and which serves as a viewer. The oil which circulates in the vicinity of the viewer has a relatively low pressure and one can evaluate the density by virtue of the number of bubbles of visible gas contained in the oil. The operator evaluates whether the fluid has been degassed by evaluating the number of bubbles which are visible to him and determines whether they are below a certain volume. It is clear that this method of verification can be only qualitative and subjective because the bubbles are generally very small, very numerous and in rapid movement. Furthermore, dissolved gases pass without being seen while the proportion between the dissolved gas and the suspended gas varies as a function of temperature. Pressure in the viewer will exert a strong effect and cannot be maintained perfectly constant.